1. Field of the Invention
This invention generally relates to electrochemical batteries and, more particularly, to a battery cathode, anode, or electrolyte that may include a halogen salt additive prior to an initial charge/discharge cycle.
2. Description of the Related Art
The demand continues for an economic means of storing the energy generated from renewable, but intermittent, solar and wind power. Energy transformational technology is expected to enable the large scale integration of renewable energy and to dramatically increase power generation and transmission efficiency. Rechargeable room-temperature batteries offer several advantages for these applications, including scale flexibility, economic maintenance, and energy-storage efficiency, as compared to other energy-storage technologies such as fly wheels, pumped water, compressed air, and high-temperature sodium/sulfur batteries. Although lithium-ion batteries have been successfully used, the demand for lithium drives concerns over its reserve and increasing cost, which renders large scale applications of lithium-ion batteries doubtful. Therefore, a low-cost rechargeable battery alternative to expensive lithium-ion batteries has been sought. Sodium-ion batteries (SIBs) are being considered as a lithium replacement candidate, because sodium has very similar properties to lithium, but at a cheaper cost.
In common with all batteries, electrolytes indispensably serve as the medium for ion transport between cathodes and anodes. Of course, the primary function of the electrolyte is to promote efficient Na+-ions transport within a rechargeable SIB. Therefore, it is critical that the electrolyte be formulated to support high ionic conductivity. Liquid electrolytes consist of a dissociable sodium salt dissolved in a solvent, thus, forming highly mobile solvated Na+-ions. In addition, to assure stability, the difference in energies between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the solvent should be larger than the difference between the anode chemical potential, μA, and cathode chemical potential, μC [1]. Clearly, if μA is above the LUMO, the electrolyte will be reduced at the anode. Conversely, if μC is below the HOMO, the electrolyte will be oxidized at the cathode. Alternatively, even if these conditions are not strictly satisfied, stability can still be achieved if reactions occur at the electrode-electrolyte interface to produce a stable solid electrolyte interphase (SEI), which prevents further reactions.
In general, the chemical potentials of SIB anode materials are higher than the energy of LUMO of electrolytes. As a results, SEI layers form on the anodes to prevent the reaction between anode and electrolyte. The stability of the SEI layers determines whether the SEI layers dissolve into the electrolyte, especially at high temperatures. Without the protection of stable SEI layers, electrolytes continually decompose on the anode surface as sodium-ions are continuously consumed in the system, leading to rapid SIB capacity degradation.
It would be advantageous if a material could be added to a battery to promote stable SEI layers and better capacity retention, and to retard the dissolution of SEI layers in electrolyte.
It would be advantageous if this material could be added to the electrolyte, cathode, anode, or a combination of these battery components.
It would be advantageous if this material could be added to the electrolyte, cathode, anode, or a combination of these battery components, prior to initially charging and discharging the battery.    [1] J. B. Goodenough, Y. Kim, Challenges for rechargeable Li Batteries, Chem. Mater. 22, (2010), 587-603.